1. Field of the Invention
The present invention relates to a novel method of carrying out Michaelis-Arbuzov or Arbuzov type rearrangements, and more particularly, it relates to the autogeneous rearrangement of tris(2-chloroethyl) phosphite to bis(2-chloroethyl) 2-chloroethylphosphonate.
2. Prior Art
Michaelis-Arbuzov reactions have long been known, having been first described in J. Russ. Phys. Chem. Soc. Vol. 38, 687 (1906). As a result of extensive investigations in the field it has come to be accepted that the Michaelis-Arbuzov reaction in its simplest form comprises the reaction with rearrangement of a phosphite with an organic halide, to obtain a phosphonate. An illustrated equation is as follows: ##STR1## wherein R is an alkyl or substituted alkyl radical, X is halogen and R' is an alkyl or substituted alkyl having at its point of attachment to X an aliphatic carbon atom.
If R and R' are identical only catalytic quantities of R'X are required to cause rearrangement of the phosphite to phosphonate. On the other hand, where R and R' are different, a stoichiometric amount of R'X is required to prepare the desired phosphonate product. In each case the rearrangement is accomplished by heating.
In certain instances, however, where an ester group of the phosphite reactant itself bears a halogen atom bonded to an aliphatic carbon atom as, for instance, a tris(2-chloroethyl) phosphite, the presence of any separate organic halide reactant is unnecessary. The reaction illustrated below is frequently spoken of as the Arbuzov rearrangement and constitutes a specialized embodiment of the generic Michaelis-Arbuzov reaction: ##STR2## where R" is an alkyl or substituted alkyl having at its point of an attachment to X an aliphatic carbon atom.
The scope of reactants available for the reaction has been defined in the course of the investigation carried out since 1906. It has come to be known that all that is critical to the reaction is:
(1) that the phosphite reactants embody a trivalent phosphorus atom that carries an ester group, therefore: ##STR3## and, that the R' group of a separate organic halide reactant or the R" group where the phosphite itself contains reactive halogen have at its point of attachment to the halogen an aliphatic carbon atom; in addition, in the instance of R", it is critical that the halogen (X) be attached to a carbon atom which is a beta carbon with respect to the oxygen moiety, or to a carbon atom which is further removed from the oxygen moiety. The moieties other than those illustrated above do not participate in this reaction and hence their identity is not critical.
For example, reaction goes forward with phosphite esters: ##STR4## with phosphonite esters: ##STR5## with the phosphinite esters: ##STR6## or expressed more generically, with any compound of the formula: ##STR7## where each R, taken separately, independently represents hydrocarbon or substituted hydrocarbon, or two R groups, taken jointly, together represents a divalent radical which with the phosphorus atom represents a cyclic unit, and each of m and n independently is an integer from 0 to 1. In the instance of phosphite and phosphonite esters wherein not all oxygen containing moieties are the same, mixtures of products will result. Not withstanding, the reaction goes forward and the resulting mixtures are conventionally separated.
It is necessary to control the process temperature and the residence time especially at high reaction temperatures to inhibit a degradation of the product. As a result, prior art batch processes generally necessitated a low reaction temperature over a long reaction time to avoid side reactions.
Prior art methods of carrying out the reaction are, for instance, by passing a thin film of liquid reaction mixture (to provide close temperature control) through a reaction zone at a temperature of 195.degree. C. to 260.degree. C. as illustrated in U.S. Pat. No. 3,483,279 to Davis et al., or by heating of the phosphite in a reactor at about 160.degree. C. for five hours with or without the use of a solvent. The latter method, however, has been found to be of low efficiency and very slow and is usually carried out in a batch-wise operation. The latter method utilizing a solvent requires further distillation in order to separate the product from the solvent used.
Tris(2-chloroethyl) phosphite is known to undergo a molecular rearrangement to form bis(2-chloroethyl) 2-chloroethylphosphonate at temperatures in the range of about 150.degree.-180.degree. C. Historically, the use of a reaction solvent has resulted in better yields than if no solvent is used.
The advantage of conducting the rearrangement in a solvent were found to be a more easily controlled exotherm due to the heating and reflux of the solvent which removes the heat of reaction, and inhibiting the degradation of the phosphonate product produced. Solvents mentioned in the prior art as being suitable for use in such rearrangement reactions were those disclosed in U.S. Pat. No. 2,725,311.
The aromatic solvents utilized in the prior art have, however, required tedious distillation procedures to separate the solvent from the product. In prior art continuous rearrangement process using heat exchangers or stirred tanks, the product containing the solvent had to be immediately removed, and continuously distilled over a period of hours to remove the solvent. In the process of stripping the solvent additional product would be lost due to the formation of degradation products. A byproduct such as ethylene dichloride from the production of bis(2-chloroethyl) 2-chloroethylphosphonate, for instance, during the vacuum distillation process would deteriorate the vacuum, and cause the temperature to increase resulting in further product decomposition. This and other routes of decomposition, of 2-chloroethyl esters of phosphoric acids are disclosed by Kafengauz et al. in Soviet Plastics, April 1967, pp. 73-75 wherein the formation of 1,2-dichloroethane, vinyl chloride, acetaldehyde and acidic residual products are discussed. In addition to the distillation difficulties, even after distillation appreciable amounts of the prior art aromatic solvent would remain in the product.
U.S. Pat. No. 2,725,311 disclose the use of solvents in the preparation of the phosphonate product by a process in which phosphorus trihalide is reacted with alkylene oxide and the temperature is raised to 150.degree. C. to cause the rearrangement. Suitable solvents were disclosed to be o-dichlorobenzene, aromatic mineral spirits, ethanol, or those solvents capable of dissolving the phosphonate which are easily volatized.
U.S. Pat. No. 4,144,387 discloses the slow addition of tris(2-chloroethyl) phosphite to a product heel (i.e, a heel of the preformed product) at a temperature below 180.degree. C. to avoid side reactions such as polymerization. The initial heel is said to be produced by the autogenous reaction of the phosphite at 175.degree. C. over a period of hours using o-dichlorobenzene as the solvent. After the initial heel was produced it was used in the subsequent production process to form additional phosphonate product without any additional solvent. This method is said to eliminate the need for using the inert solvent in the final product run, which would necessitate expensive solvent separation.
The object of the invention disclosed herein is to provide a means whereby the thermal isomerization of the phosphite can be accomplished to form the phosphonate product, without the necessity of further distillation to remove the solvent.
Another object of the invention is the production of high quality bis(2-chloroethyl) 2-chloroethyl phosphonate by the rearrangement of tris(2-chloroethyl) phosphite using a solvent which can be conveniently and economically removed from the product.
Other objects of the invention or advantages of the invention will be evident from the following disclosure.